Low dimensionality enhances quantum fluctuations, triggering novel states of quantum matter to emerge. In real materials, low dimensionality usually arises from spatially strongly anisotropic couplings. Here, we demonstrate a different mechanism: in two-dimensional systems with coupled alternating ferromagnetic (FM) and antiferromagnetic (AFM) spin-
1/2 chains, an applied magnetic field may drive a dimensional reduction. Under magnetic field, the FM chains polarize and stiffen, suppressing the propagation of transverse AFM fluctuations from one chain to another, and effectively induce one-dimensional behavior at low energies. For a model describing botallackite, Cu
2(OH)
3Br, quantum Monte Carlo dynamics show that beyond a critical magnetic field, the low-energy spectrum reduces to that of a one-dimensional AFM Heisenberg spin-
1/2 chain with field-dependent incommensurate two-spinon fluctuations, providing clear signatures for inelastic neutron scattering.
In the quasi-one-dimensional antiferromagnets CsMnBr
3 and RbMnBr
3 the magnetization produced by an external magnetic field displays a much stronger dependence upon the direction of the field than expected from calculations done for a system of classical spins at zero temperature. By performing these calculations for a system at the temperature at which experiments were made it has been shown that thermal fluctuations may account for the observed anisotropy of the magnetization. We analyze here the role of quantum fluctuations. We find that the spin-wave approximation does not give meaningful results when applied to these systems and explain why this is so. We perform then density-matrix renormalization-group calculations, which show that the anisotropy is enhanced over its classical value by quantum fluctuations already at
T=0.